Because of their chemical resistance, physical properties, ease of processing, and ability to adhere well to a wide variety of substrates, fillers, and reinforcing agents, epoxy resins have been used for over forty years for such applications as composites, adhesives and sealants, filament winding, potting compounds, and the like. By definition, any molecule containing the epoxy group ##STR1## can be called an epoxy. Many commonly used epoxy resins are based on the glycidyl group ##STR2## which can be introduced into a molecule containing a hydroxyl --OH or amine --NH.sub.2 by reaction with epichlorohydrin; for example, the diglycidyl ether of bis(hydroxyphenyl)propane, triglycidyl p-aminophenol, tetraglycidylmethylene dianiline, and the like. Also in use are glycidyl ethers of phenol-formaldehyde condensates (novolacs). Another route to epoxy resins is the epoxidation of olefinic unsaturation in compounds such as the cyclohexenylmethyl ester of cylcohexene carboxylic acid. In the formulation of an epoxy resin system, two properties of the epoxy resin are essential--the epoxy functionality (that is, the number of epoxy groups per molecule) and the epoxy equivalent weight (the number of grams of epoxy resin which contain one chemical equivalent of epoxy group). As an example, triglycidyl p-aminophenol has an epoxy functionality of three (three epoxy groups per molecule) and an epoxy equivalent weight of 110. Functionality and equivalent weight are concepts familiar to any chemist.
Epoxy resins can be homopolymerized to polyethers by the use of Lewis acid catalysts such as boron trifluoride and Lewis base catalysts such as tertiary amines or ethyl methyl imidazole. These catalysts are conventionally referred to as epoxy curing agents or hardening agents or simply hardeners, and are used in relatively low concentrations relative to the epoxy resin. It is more common, however, to employ hardeners which actually react with the epoxy group and become a part of the final cured solid epoxy resin. The most commonly employed "co-reactant" type hardeners are the diamines and polyamines such as diethylenetriamine and methylene dianiline, and the carboxylic acid anhydrides such as phthalic anhydride, methyl tetrahydrophthalic anhydride, and methyl bicycloheptenecarboxylic acid anhydride (commonly known as "nadic methyl anhydride"). Just as one refers to the epoxy equivalent weight, one refers to an amine equivalent weight or an anhydride equivalent weight--the number of grams of hardener which contain one chemical equivalent of amine hydrogen or anhydride respectively. In theory, one epoxy equivalent weight will react completely with one hardener equivalent weight to form the final solid cured resin. This is referred to as a stoichiometry (or stoichiometric ratio) of one to one. In actual practice, the epoxy resin component will often be used in excess of the amount predicted from the epoxy and hardener equivalent weights--for example, a stoichiometry of 0.8 equivalents of hardener to 1 equivalent of epoxy resin.
It is common practice in the art to separate the components of an epoxy resin system into two parts--a "Part A" containing the epoxy resin or resins, and a "Part B" containing the hardener or hardeners. This approach has two advantages. Curing of the epoxy resin system cannot begin until the epoxy resin and the hardener are mixed--in this two-part form, the system is indefinitely stable. Moreover the amounts of epoxy resin in "A" and the amounts of hardener in "B" are chosen by the supplier to provide the desired stoichiometry once they are blended together by the customer. This latter advantage is obviously useful for those customers who are not themselves chemists, and might have difficulty calculating equivalent weights.
Once the epoxy resin and hardener are mixed, reaction will begin, leading eventually to a solid resin. The reaction however may proceed extremely slowly, especially when anhydrides are being employed as the hardeners. In order to speed up sluggish reactions, catalysts called accelerators are often added in small amounts. Tertiary amines such as benzyl dimethylamine and Lewis bases such as ethyl methyl imidazole are useful accelerators for epoxy/anhydride systems. A relatively new family of accelerators, the cyclic thioureas (also known as dialkyl imidazole-2-thiones) have proven extremely effective in accelerating epoxy/anhydride cures even at mild or ambient temperatures.
However, even with the addition of accelerators, it is customary to use heat in advancing the cure of epoxy resin systems. Often cures are carried out in stepwise fashion--the mixture is heated at a relatively low temperature at first, followed by subsequent heating at one or two higher temperatures. In order to determine the optimum cure cycle for a given epoxy resin system, a variety of laboratory techniques can be used to follow the progress of the polymerization and determine when cure is complete. Among these tests are differential scanning calorimetry (DSC), thermal mechanical analysis (TMA), dielectric analysis, and infrared analysis (IR). Once a satisfactory cure cycle has been worked out for a specific system, it is usually unnecessary to repeat the laboratory testing with each subsequent batch that is to be cured. Normally the supplier of the epoxy resin system can supply customers both with suggested ratios of epoxy resin to hardener and with suggested cure cycles which the customer can carry out without the need for extensive laboratory testing.
In addition to the epoxy resin, hardener, and accelerator, it is known in the art to include minor amounts of other ingredients in an epoxy resin system to facilitate processing or improve final properties. Among said minor ingredients are "bubble breakers" (defoamants), leveling agents, supplemental accelerators, and the like. Selection of said components for such purposes is considered within the skill of the ordinary worker.